[sidebar]Unless you get adequate vitamin D, about half the bone-building calcium you take ends up down the toilet, notes Felicia Cosman, MD, clinical director of the National Osteoporosis Foundation. How much vitamin D is enough? “You should get 400 IU every day—and 600 after age 70—from a multivitamin or calcium-plus-D supplement,” says Dr. Cosman, who is an associate professor of clinical medicine at Columbia University. Some people might need up to 1,000 IU per day, depending on their current vitamin D levels. Cosman recommends having your doctor do a blood test to make sure you’re not vitamin D deficient if you have osteoporosis, low bone-mineral density, or vitamin absorption problems (e.g., from celiac disease), or if you’re premenopausal and have a history of breaking bones easily.  Quick tip Forget exposing unprotected skin to the sun to produce vitamin D. Food and supplements can provide all you need. More from Prevention: The Best Food Sources Of Calcium